Several years ago, I had junky trees cut down that were shading my vegetable garden. To my surprise, brambles and briars grew up from nowhere.

I can cut them down, but I don't want to keep fighting them every year. So my plan is to plant the area with ornamental trees like Sourwoods, Star Magnolias, and Korean Dogwoods.

Those won't grow tall enough to shade my garden, but they should shade the ground around them to prevent the briars and brambles from regrowing.

Or so I hope. Is this a reasonable strategy? It is a half-shade area.

It's worth a try; if nothing else, you'll have ornamental trees to enjoy while you're weeding
You might still have volunteer brambles popping up for a while, though. I know that Smilax rotundifolia (Greenbrier), for one, grows vigorously even in part shade.

I've also experienced this problem - mulberry trees suddenly appeared out of nowhere. In one area I'm in the process of converting the space (which is anchored by an ancient Black Walnut ) into a shrub border. Mulch is my friend at this point. The understory trees and shrubs have not shaded out the unwanted scrub plants - this year, due to the record rains - we ended up with tons of poison ivy! Under the walnut we planted red buds, star magnolias, ninebark, japanese maples and a number of spreading evergreen shrubs. Hoping that repeated weeding, lots of mulch and maturing plants will smother the weeds. Good Luck on your project.

Plants will always find bare soil and fill it fast given a little sunlight. The bad thing about blackberries and other brambles is once they start growing it's hard to shade them out. I have a few growing in full shade.

I'm planning to cut the brambles down to ground level and mowing over them until they die. The roots can't live forever. I can't think of a better plan.

Sounds like a good plan. Sourwoods are one of my favorite trees! Mowing works.

Do you have an aversion to glyphosate?
If not: It is labor intensive but what I do is cut each stem to between .5 to 1"and squirt a little full-strength gly and it's gone forever.
Sawbriers, brambles, perennial morning glorys, any weed tree.
I always carry these in my pockets.

Blackberries come up as soon as the shade is removed at my house too.
I think the plan of putting the shade back is a sound one.
I would avoid poison, though.
If you get enough rain to get berries, that would be a nice bonus.
Too dry at my house... The berries dry up into husks without ever turning ripe.
The plant is good medicine...
I recently dug a buncha blackberry roots for an aquaintence who had intestinal issues .... Just like the lady in the ad on tv.
You can't purchase meds as good as this.

Muddy1 said:It's worth a try; if nothing else, you'll have ornamental trees to enjoy while you're weeding

LOL!, Thank you for that. Made my day! Fighting with brambles under Sourwoods and Dogwoods is sure better that fighting with brambles without them!